Journal of geomagnetism and geoelectricity最新文献

{"title":"Magnetotelluric Phase Anisotropy above Extensional Structures of the Neogene Pannonian Basin","authors":"A. Ádám","doi":"10.5636/JGG.49.1549","DOIUrl":"https://doi.org/10.5636/JGG.49.1549","url":null,"abstract":"Neogene extensional forces generated narrow rift zones (deep subbasins) in the Pannonian Basin, among others one of the deepest, the 7 km deep Bekes graben. Above this structure strong magnetotelluric (MT) phase anisotropy (phase-deviation in two orthogonal directions) has been observed indicating the upwelling of the partially molten asthenosphere. This upwelling is certainly the first to prove the validity of the deep mantle structure of Buck's extensional narrow rift model by an electromagnetic method. A geotectonic relation will also be pointed out between the causes of MT phase anisotropy in cratonic and orogenic belts.","PeriodicalId":156587,"journal":{"name":"Journal of geomagnetism and geoelectricity","volume":"16 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1997-11-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"115149614","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Experimental Design for Surface-to-Borehole Hydrocarbon Applications","authors":"B. Kriegshäuser, A. Tripp, L. Tabarovsky","doi":"10.5636/JGG.49.1659","DOIUrl":"https://doi.org/10.5636/JGG.49.1659","url":null,"abstract":"We discuss in a simplified surface-to-borehole example the resolution of the coefficient of anisotropy λ for an anisotropic layer sandwiched in an isotropic half-space as a function of transmitter-receiver configuration and magnetic field components. Our analysis supposes that the horizontal resistivity of the anisotropic bed has been established by previous logging, and that only horizontal magnetic fields will be used to determine the vertical resistivity. The resolution analysis shows that for the model studied the coefficient of anisotropy λ can be resolved within 10% for a broad range of transmitter-receiver configurations if ∂B x /∂t and ∂B y /∂t are used jointly in the interpretation. Using only ∂B x /∂t or ∂B y /∂t individually, λ can only be resolved for a limited range of transmitter-receiver configurations. A joint interpretation of both horizontal magnetic field components facilitates a better resolution of the coefficient of anisotropy compared to a single interpretation.","PeriodicalId":156587,"journal":{"name":"Journal of geomagnetism and geoelectricity","volume":"17 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1997-11-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"125453420","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The Longest Conductivity Anomaly in the World Explained: Sulphides in Fold Hinges Causing Very High Electrical Anisotropy","authors":"A. Jones, T. Katsube, P. Schwann","doi":"10.5636/JGG.49.1619","DOIUrl":"https://doi.org/10.5636/JGG.49.1619","url":null,"abstract":"After almost three decades of study, from its initial discovery in the 1960s to laboratory analyses of rock samples last year, we can now identify the most probable cause of the North American Central Plains (NACP) conductivity anomaly for much of its 1, 500-km strike extent. Tectonic processes operating during Paleoproterozoic Trans-Hudson orogenesis, with closure of the 5, 000-km-wide Manikewan ocean, included subduction and compression of sediments deposited during a hiatus in volcanism as the first of the advancing arcs approached the Archean continental margin to the west (Wyoming and Rae/Hearne cratons). These sediments were folded, and syngenetic sulphides within them migrated to concentrate along fold hinges, preferentially along strike, leading to high anisotropy in electrical conductivity (over 2-3 orders of magnitude). Mapping of the anomaly in similar tectonic environments, from the southern Dakotas to northern Manitoba, suggests that these processes were active along the whole western and northern margin of the orogen. However, other processes, possibly invoking graphitic emplacement in a foredeep, more likely account for the southern terminus of the anomaly from the Black Hills to southeastern Wyoming.","PeriodicalId":156587,"journal":{"name":"Journal of geomagnetism and geoelectricity","volume":"49 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1997-11-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"129195758","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"High-Performance Three-Dimensional Electromagnetic Modelling Using Modified Neumann Series. Anisotropic Earth","authors":"O. Pankratov, A. Kuvshinov, D. Avdeev","doi":"10.5636/JGG.49.1541","DOIUrl":"https://doi.org/10.5636/JGG.49.1541","url":null,"abstract":"We have developed a modified Neumann series (MNS) technique to solve Maxwell's equations for three-dimensional anisotropic earth. We assume that both conductivity and dielectric permittivity are 3 × 3 matrices, elements of which are complex-valued functions of space coordinates and frequency, whereas the magnetic permeability is symmetric 3 × 3 matrix with elements being real-valued functions of depth. Both conduction and displacement currents are taken into account. In order to derive the MNS solution, we impose the positiveness condition of Joule losses inside anisotropic earth. The MNS solution so obtained is valid for any frequency range and for arbitrary three-dimensional anisotropic earth.","PeriodicalId":156587,"journal":{"name":"Journal of geomagnetism and geoelectricity","volume":"76 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1997-11-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"131602246","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Electromagnetic Monitoring of Miyake-jima Volcano, Izu-Bonn Arc, Japan: A Preliminary Report","authors":"Y. Sasai, J. Zlotnicki, Y. Nishida, P. Yvetot, P. Morat, H. Murakami, Yoshikazu Tanaka, Y. Ishikawa, S. Koyama, W. Sekiguchi","doi":"10.5636/JGG.49.1293","DOIUrl":"https://doi.org/10.5636/JGG.49.1293","url":null,"abstract":"Miyake-jima Island, about 150 km south of Tokyo in Izu-Bonin Arc, is one of the most active basalt volcanoes in Japan. Big eruptions took place in 1940, 1962 and 1983. In this volcano, magma ascends towards a depth of a few km below the summit without any significant earthquakes or deformation, then gives rise to flank fissure eruptions because of the blockaded vent just beneath the summit crater. Hence eruption forecasts are very difficult to make with mechanical methods (i.e., seismic and deformation measurements) alone. We have developed an electromagnetic monitoring system of the volcano that combines magnetic, resistivity and electric field (SP) measurements. We expect that magma injection and the hydrothermal materials dispatched from it will result in thermal demagnetization, resistivity change and SP variations together with the electrokinetic-magnetic effect. Since October, 1995, we have continuously operated eight well distributed proton magnetometers over the island as well as two SP measurement systems on the NE and SW fissure zones. SP surveys brought to light distinct anomalies, which strongly suggest a close relation to the eruption mechanism. They are a positive anomaly up to 700 mV centered around the summit, and two negative ones amounting to -250 mV on the north and -100 mV on the southwestern mountainside. These anomalies can originate from a common mechanism: Rainwater penetrates from fissure zones along fractures toward the center of the volcano, a few km deep, where it is warmed by the heat supplied from deep-seated magma to rise through the summit vent. The down flow makes the negative, while the upwelling the positive SP anomalies, respectively. Miyake-jima island is located near the path of the Kuroshio, the most dominant ocean current in the western Pacific. A large magnetic variation amounting to several nT was observed to result from the meander of the flow path. This phenomenon produces serious EM noise and complicates monitoring for volcanic activity. It is crucial to investigate the characteristics of motionally-induced EM fields and to properly eliminate their effects.","PeriodicalId":156587,"journal":{"name":"Journal of geomagnetism and geoelectricity","volume":"19 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1997-11-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"125240603","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Geophysical challenges in using large-scale ocean-generated EM fields to determine the ocean flow","authors":"R. Tyler, T. Sanford, J. Oberhuber","doi":"10.5636/JGG.49.1351","DOIUrl":"https://doi.org/10.5636/JGG.49.1351","url":null,"abstract":"For some time, oceanographers have used measurements of the small electromagnetic fields generated by the flow of the electrically conductive oceans through the Earth's main magnetic field to infer values for the ocean flow velocities. An overview of the process of motional induction is given including a description of the global electromagnetic fields generated by a global model ocean. We describe how electromagnetic methods are currently used in oceanography and outline the most important challenges presently faced.","PeriodicalId":156587,"journal":{"name":"Journal of geomagnetism and geoelectricity","volume":"48 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1997-11-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"134453069","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"MT Forward Modeling of 3-D Anisotropic Electrical Conductivity Structures Using the Rayleigh-Fourier Method","authors":"P. Martinelli, A. Osella","doi":"10.5636/JGG.49.1499","DOIUrl":"https://doi.org/10.5636/JGG.49.1499","url":null,"abstract":"We present an algorithm for modeling the magnetotelluric response of three-dimensional multilayered structures with irregular interfaces. In this formulation, based on a Rayleigh-Fourier technique, the effect of vertical anisotropy in the electrical conductivity has also been included. This method has an applicability range complementary to other solutions based on finite differences or on integral equations, which are especially adequate to model localized bodies intruded in a host medium. To test the method, the MT response of a simple conductive structure was modeled and compared with the solutions obtained using integral equations. A good agreement between the results with similar processing times have been observed. Finally, the effect of anisotropy was estimated for the particular case of a conductive basin, showing a non-negligible contribution, depending on the relation between the vertical and horizontal values of the conductivity.","PeriodicalId":156587,"journal":{"name":"Journal of geomagnetism and geoelectricity","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1997-11-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"115080247","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"An Integral Sign Conductor in the Frontal Himalaya Region and Its Tectonic Interpretation","authors":"C. Reddy","doi":"10.5636/JGG.49.1649","DOIUrl":"https://doi.org/10.5636/JGG.49.1649","url":null,"abstract":"A 3-D thin sheet model is developed synthesizing the data from different small array studies in the frontal Himalaya region. Most of the regional anomaly is simulated by a single highly conducting zone approximating an integral sign in shape (∫). The integral sign conductor (ISC) is basically an extension of Trans-Himalayan Conductor at the northern and southern ends towards east and west respectively. The conductance map indicates that the crust beneath the Indo-Gangetic Plains (IGP) lying on the western side of the ISC is electrically one order more conducting than on the eastern part. The synthesis of many geophysical signatures has permitted to infer that the Indian shield contiguous to the frontal Himalaya is mosaic of two crustal blocks of contrasting geophysical properties. In such tectonic scenario, the ISC is seen as a northeast continuation of the Aravalli range beneath the IGP and is interpreted to represent accretion zone resulting from the collision or underthrusting of the eastern crustal block beneath the western block. The clustering of epicenters along the ISC marks the continued activation of this ancient structure.","PeriodicalId":156587,"journal":{"name":"Journal of geomagnetism and geoelectricity","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1997-11-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"124387619","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"High-Performance Three-Dimensional Electromagnetic Modelling Using Modified Neumann Series. Wide-Band Numerical Solution and Examples","authors":"D. Avdeev, A. Kuvshinov, O. Pankratov, G. Newman","doi":"10.5636/JGG.49.1519","DOIUrl":"https://doi.org/10.5636/JGG.49.1519","url":null,"abstract":"We present a new, accurate, high-performance, wide-band three-dimensional (3-D) solver for the electromagnetic (EM) field scattering problem in an isotropic earth. The solver relates to those based on the volume integral equation (IE) approach and exploits a modified Neumann series (MNS) technique to solve Maxwell's equations. The solver allows for the conduction, polarization and displacement currents to be taken into account and admits for 3-D earth excitation by arbitrary electric or/and magnetic sources. We estimate the solver efficiency for scatterers discretized into Nx × Ny × Nz prisms, where it requires only about 6NxNyNz (log2(2Nx) log2(2Ny) + 6Nz) multiplications to get one term of the MNS expansion and about 200 NxNyNz2 bytes of memory. Our experience show that the number of terms N which are to be summed up to get the solution to 1% accuracy doesn't exceed fifty for the models with the conductivity contrast of up to 100. We demonstrate the solver versatility for magnetotellurics (MT) and controlled-source simulations. EM fields arising from a 3-D model with two high-contrast thin layers residing in layered earth were simulated due to a 10 Hz electric dipole located at the surface. When the layers were discretized into 16, 384 prisms our code on a Pentium-100 MHz took T ∼ 58 minutes, M ∼ 7 Mbytes and N ∼ 280. We also modeled the 0.1 Hz and 0.01 Hz MT responses within 3-D model with 1 Ω·m and 100 Ω·m blocks. When the blocks were discretized into 8, 000 prisms the code took T ∼ 5 minutes, M ∼ 8 Mbytes, and N ∼ 25. Finally fields for a crosswell model including a 3-D conducting target were simulated for 0.1 kHz and 10 kHz electric and magnetic dipoles in the wellbores. While the target was discretized into 6, 250 prisms the code took T ∼ 16 minutes, M ∼ 13 Mbytes, and N ∼ 24. All simulations showed from very good to excellent agreement with those of the other 3-D solvers.","PeriodicalId":156587,"journal":{"name":"Journal of geomagnetism and geoelectricity","volume":"49 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1997-11-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"129035571","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Robust Processing of Magnetotelluric Data from the Auroral Zone","authors":"X. Garcia, A. Chave, A. Jones","doi":"10.5636/JGG.49.1451","DOIUrl":"https://doi.org/10.5636/JGG.49.1451","url":null,"abstract":"Magnetotelluric (MT) data acquired during September-October, 1994, in northern Canada, were strongly influenced by non-uniform source field contributions from the auroral electrojet, and especially by intense auroral episodes. The largest effect on the estimate of the magnetotelluric impedance tensor elements was during intervals of highest magnetic activity, which primarily correlated with high auroral activity and was observed during local nighttime. In comparison, during the day the effect on the normal magnetotelluric impedance tensor response was usually, but not always, small. A robust controlled-leverage processing algorithm was applied to these data in an attempt to extract the stable uniform field estimates of the impedance. The differences between nonrobust and robust processing of the entire data set is compared to that obtained after dividing the time series into daytime and nighttime segments. The nonrobust estimate using all data is controlled by the nocturnal data, which are, in turn, dominated by non-uniform source effects. However, nonrobust processing of only the daytime data fails to recover a useful result. There is little difference between the robust response for the entire and daytime data provided that the fraction of auroral activity is not large, i.e., in excess of half of the available data series. In addition, examination of the time-dependence of the response functions shows that the strongest bias is observed during the initial quarter of an auroral event.","PeriodicalId":156587,"journal":{"name":"Journal of geomagnetism and geoelectricity","volume":"6 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"1997-11-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"131836449","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}